This invention relates generally to the field of temperature threshold sensing; and, in particular, to a multiple temperature threshold sensing circuit having a single sense element.
Over the past two decades, the semiconductor industry has greatly advanced from incorporating a few transistors on one integrated circuit chip to incorporating millions of transistors. The integrated circuit is at the heart of most electronic equipment today, e.g. navigational systems, computers, pocket calculators, industrial monitoring and control systems, digital watches, digital sound systems, word processors, communications networks, and innumerable others. The vast number of transistors on a small area of semiconductor material has its advantages in speed, reliability, and negligible weight, but has its disadvantages in power consumption. More specifically, due to the increase in power consumed by each transistor, there exists a cumulative effect of temperature rise.
Conventional systems use two forms of cooling systems: passive and active. These cooling systems are mounted a circuit board that includes the integrated circuit package. Passive cooling involves the use of a heat sink. This form of cooling however has limited capacity to dissipate heat and increases the weight of the complete circuit board module. Active cooling involves the use of a device such as a fan which pulls air over the package to cool the die. Fans are not efficient because they require more space and power. In addition, fans are not desirable because they generate noise.
In addition to the use of cooling systems, thermal sensing systems are used to monitor the temperature of the integrated circuit. More specifically, they monitor portions of the integrated circuit having specific functions within an electronic system to determine when the temperature exceeds a predetermined temperature threshold. Once the integrated circuit has exceeded the predetermined temperature threshold, that particular portion of the integrated circuit having the specified function is shut down. One such thermal sensing system comprises a thermocouple attached to a heat sink. Another thermal sensing system includes a diode or a bipolar transistor and an external analog circuit. Since the current and voltage characteristics of a diode are temperature dependent, the external analog circuit is used to track the current and voltage characteristics of the diode, while simultaneously monitoring the temperature of the diode. Once the temperature has exceeded a particular value, the external analog circuit generates a signal to trigger the shutting down of that particular function of the electronic system. For a sensing system which includes a bipolar transistor in lieu of the diode, the external analog circuit monitors the base-emitter voltage Vbe of the transistor since the reference voltage of the bipolar transistor is temperature dependent.
Some thermal sensing systems include hysteresis logic. One such implementation may include a current mirror coupled to a thermal sense element, such as a diode or bipolar transistor. A current reference, having a current proportional to the absolute temperature, generates a bias current which is applied to the current mirror. The bias current and resistance within the thermal sensing circuit is predefined such that the thermal sense element conducts current at a particular predetermined temperature threshold. A typical pre-determined shutdown threshold may be 150xc2x0 C. In the instance where the thermal element is a bipolar transistor, as noted above, the reference voltage of the bipolar transistor is quite temperature dependent. Its base-emitter voltage Vbe has a negative temperature coefficient of approximately 1.5 to 2.5 mV/xc2x0 C. As temperature increases, the base-emitter voltage Vbe necessary to turn on the bipolar transistor decreases. When the temperature of the circuit reaches the predetermined temperature threshold, the bipolar transistor begins to conduct current. The hysteresis logic switches in additional current applied to the base of the bipolar transistor after the predetermined temperature threshold is reached.
This thermal sense system for protecting an integrated circuit from overheating is successful; however, in many systems, it is desirable to keep all functions available to the end-user. Setting a thermal sense flag eliminates the need to shut down the function entirely. A second thermal sense circuit with a lower detect threshold creates this feature. However, utilizing separate sense elements introduces a semiconductor matching problem where the coefficients associated with the process, die stress, and thermal gradients have a high probability of not being equivalent. In addition, adding a second thermal sense circuit requires a greater portion of die area to incorporate redundant current mirroring required for multiple circuits. This problem escalates when sensing more than two distinct thresholds. Thus, there is a need for a thermal sensing system that utilizes one thermal sensing element having the capability to sense two or more distinct thresholds.
A temperature sensing system provides at least one detect signal related to temperature in an integrated circuit. This system uses one analog thermal sensing circuit to detect two or more temperature thresholds and differentiates the temperature thresholds using a time multiplexed logic control. The system includes the thermal sensing circuit and a decode circuit with at least two detect latches. Optionally, the system may include a hysteresis circuit. This thermal sensor circuit, connected to the integrated circuit, generates a detect signal in response to a temperature selection signal. This flexible on-board temperature monitoring solution reduces the cost of thermal feedback sensing by reducing die area and improves the correlation of detected temperatures. This solution reduces the possibility of mismatch and tracking errors between two or more sense elements.
There are numerous advantages of the present apparatus of sensing temperature at multiple thresholds over previous implementations. First, the use of a single sense element minimizes the possibility of mismatch and tracking errors between two or more sense elements. This implementation reduces the relative error to current mirror mismatch. The simple time multiplexed decode of the detect signal saves die area verses the implementation of redundant sense circuits; thus, it provides a cost effective solution. Most importantly, with the addition of relatively few components, a time multiplexed method of sensing die temperature results in a flexible on-board temperature monitor.